This application relates to a control system and method for use in a hybrid power supply apparatus comprising a current generating device, such as a fuel cell, and an energy storage device, such as a battery. The invention ensures that the fuel cell operates in a quasi-steady state mode irrespective of fluctuations in load power demands. By restricting the operation of the fuel cell to discrete current output modes with minimal state changes, the useful service life of the fuel cell is prolonged.
Hybrid power supply systems comprising a current generating device, such as a fuel cell, and an energy storage device, such as a battery, are well known in the prior art. In a hybrid system the fuel cell is used to charge the storage battery which in turn supplies power to a load on an xe2x80x9con-demandxe2x80x9d basis. Alternatively, the fuel cell and the battery may jointly supply power to the load depending upon the power requirements.
Hybrid power systems offer several advantages. Many fuel cell systems include fuels processors such as reformers for converting conventional fuels to hydrogen or hydrogen-enriched gas for use by the fuel cell. In general, the combination of a fuel cell and a reformer makes it difficult to respond quickly to variations in external load since the response time of the reformer is slow. This is particularly the case for loads such as electric lift vehicles which have a pattern of power usage or xe2x80x9cduty cyclexe2x80x9d which is characterized by loads which fluctuate substantially during the course of a work shift. The addition of a charged energy storage means enables the hybrid system to respond quickly to power demand surges, while maintaining the advantages of a fuel cell system including extended operating times, low emissions and the flexibility to utilize many readily available fuels.
Hybrid power supply control systems are known in the prior art for use in applications subject to sudden load fluctuations. U.S. Pat. No. 4,883,724, Yamamoto, issued Nov. 28, 1989 relates to a control unit for a fuel cell generating system which varies the output of the fuel cell depending upon the state of charge of the battery. In particular, a DC/DC converter is connected between the output of the fuel cell and the battery and is responsive to a control signal produced by a controller. The purpose of the Yamamato invention is to ensure that the storage battery is charged for recovery within the shortest possible time to reach a target remaining charge capacity under charging conditions that do not cause deterioration of performance of the battery. When the charged quantity of the battery is recovered to the target value, the controller lowers the output of the fuel cell to its normal operating state. In the case of no external load, such as during extended periods of interruption in the operation of the lift truck, the fuel cell is controlled to stop after the storage battery is charged.
The primary limitation of the Yamamoto control system is that the control algorithm is designed for minimizing the recharge time of the storage battery rather than prolonging the useful life of the fuel cell. By varying the fuel cell output to charge the storage battery for recovery within the shortest possible time, the long-term performance of the fuel cell is compromised. The need has therefore arisen for an improved hybrid control system and method which preserves near steady state operation of the fuel cell while avoiding both over-charge and over-discharge of the battery.
U.S. Pat. No. 4,839,574, Takabayashi, also discloses a generator system utilizing a fuel cell and a reformer. Depending upon the state of charge of the battery the output of the fuel cell may be adjusted in a stepwise fashion. In the Takabayashi system the amount of raw material supplied to the reformer is maintained constant within a range of charged energy to ensure stable operation of the reformer. However, depending upon the state of charge of the battery and the load demands, the fuel cell and the reformer may be subject to frequent adjustments.
As has been shown in the prior art, it is desirable to choose discrete charging currents corresponding to the specific states of charge of the battery in order to operate the fuel cell (or other current generating device) in a steady mode. However, this approach can result in unstable operation when used with a dynamic load such as a hybrid vehicle. The result can be cycling between states. The purpose of this invention is to introduce a control scheme which will ensure that the current generating device can run in a near steady mode in the presence of a dynamic load.
In accordance with the invention, a method of controlling the charging characteristics of a hybrid power supply apparatus comprising a current generating device and an energy storage device connectable to an external load is disclosed. The method comprises the steps of:
(a) repeatedly determining the state of charge of the energy storage device;
(b) operating the current generating device during an operating period to produce an output current for delivery to the energy storage device;
(c) repeatedly comparing the state of charge determined in step (a) to a predetermined set of target state of charge set points for the energy storage device stored in memory; and
(d) adjusting the output current of the current generating device by a predetermined increment whenever the state of charge of the energy storage device approximates one of the target state of charge set points, wherein said predetermined increment is sufficiently large in magnitude such that the frequency of adjustment of the output current is minimized during the operating period notwithstanding fluctuations in the power demands of the load.
The current generating device may comprise a fuel cell. Preferably the current generating device produces a DC output current and the DC output current is delivered from the current generating device to the energy storage device via a DC/DC converter. The fuel cell output current is preferably adjusted by controlling the operation of a fuel processor delivering fuel to the fuel cell. The method may further include the step of calculating the predetermined increment according to a control algorithm such that the time period between adjustments of the output current is not less than a minimum time T. The method may further include the step of dynamically storing the state of charge set points during the operating period.
A hybrid power generating system is also disclosed for implementing the above method, the system comprising:
(a) an energy storage device connectable to a load;
(b) a current generating device for producing a charging current during an operating period;
(c) a detector for determining the state of charge of the energy storage device; and
(d) a controller for controlling the operation of the current generating device, wherein the controller receives input from the detector and comprises means for repeatedly comparing the state of charge of the energy storage device to a predetermined set of target state of charge set points stored in memory, wherein the controller adjusts the charging current of the current generating device in predetermined discrete increments whenever the measured state of charge of the energy storage device approximates one of the target set points, the increments being sufficiently large in magnitude such that the frequency of adjustment of the charging current is minimized during the operating period notwithstanding fluctuations in the load.
In one embodiment of the invention Applicant""s method may include the steps of:
(a) repeatedly determining the state of charge of the energy storage device;
(b) operating the current generating device during an operating period to produce an output current for delivery to the energy storage device;
(c) storing in a memory a set of target state of charge set points for the energy storage device, the set points defining a plurality of state of charge intervals;
(d) storing in a memory a plurality of charge rates for the output current, each of the charge rates corresponding to at least one of the state of charge intervals;
(e) repeatedly comparing the state of charge determined in step (a) to the set of target state of charge set points to assign one of the state of charge intervals; and
(f) setting the output current of the current generating device to a charge rate corresponding to the state of charge interval assigned in step (e).
In a preferred embodiment the minimum time T is at least an order of magnitude larger than the time required to adjust the output current from one of the charge rates to another one of said charge rates.